This invention relates to the punches used to pierce holes in hydroformed parts and more particularly to the sealing of the punches with respect to the hydroformed parts.
As is well known in the art of hydroforming parts from a tubular metal part, the hydroforming pressure used to form the part can also be utilized to assist a punch in piercing the part to produce a required hole therein during the hydroforming process. Thus eliminating the need for a secondary operation such as laser cutting to form the hole in an internally unsupported region of the part. For example, hydroforming apparatus including such a punch is disclosed in U.S. Pat. No. 5,398,533 assigned to the assignee of this invention.
Another example of piercing a hydroformed part is shown and labeled as xe2x80x9cPrior Artxe2x80x9d in FIG. 1 of the accompanying drawings and is included to help illustrate a problem in the piercing phase of the hydroforming process that can result from using current state of the art punches. Referring to FIG. 1, the piercing operation is performed following completion of the hydroforming of a hollow metal part 10 in a die cavity 12 formed by dies 14 and 16 and while the formed part remains in intimate contact with the surface of the die cavity and while the hydroforming liquid fluid 18 in the part remains at the high pressure required for forming the part. In the piercing operation, a punch 20 of conventional cylindrical configuration whose piercing end is initially held in a position so as to form a continuation of the die cavity surface is then forced to pierce the formed part as shown to produce a hole required in the part. In this example, the required hole has a round or circular shape and the punch accordingly has a right-circular cylindrical surface.
When the punch 20 is forcibly extended for piercing, the forming pressure in the part supports the formed part about the periphery of the punch allowing the piercing or shearing action of the punch to occur. And the punch shears a slug 22 in forming the required hole in the part wherein the slug falls as a loose piece and lays inside the formed part for later removal.
However, when the punch pierces and enters the formed part, it has been found that considerable leakage can occur past the punch causing the forming pressure in the part to fall to the point where it fails to adequately support the part internally for the piercing action by the punch. When this occurs, the part can collapse inwardly in the region being pierced resulting in a undesired misshaped part at the end of its processing in the hydroforming die cavity. This is shown with phantom lines in FIG. 1. Moreover, in the case where several holes are to be pierced in the formed part in similar manner with other like conventional punches, the potential leakage problem increases proportionally and there is even more likelihood of ending up with misshaped parts.
Punches of various sizes and shapes are used to produce the holes required in hydroformed parts. With one of the main reasons for the holes being to provide access for tooling such as welding tips and wherein it may be necessary for the welding tips or other tooling to enter at an angle. Attempts have been made at solving the leakage problem with a punch configuration that extrudes a substantial portion of the formed part inwardly along the side of the punch to thereby provide improved sealing. Wherein such metal extrusion can be considerably longer and thus deeper than the extent of the extruded portion 24 shown in FIG. 1 that typically occurs without such a leakage solving punch modification. And the extent to which such extrusion is required to effect the necessary sealing can require a long deep extrusion into the formed part that is not acceptable such as in the case where it would create an obstruction to the required entry of tooling.
The present invention solves the above leakage problem in a very simple manner with a new and improved cost-effective punch configuration. Wherein during the hole forming operation, the punch configuration rolls over an annular edge portion of the formed part that defines the hole with the resulting rolled edge portion forcing a tight high-pressure seal between the part and the punch. And depending on the particular hole requirements and the sealing pressure capacity required to prevent leakage, the rolled edge portion of the formed part is formed by the punch configuration so as to extend only partially or completely about the punch in order to effectively prevent high pressure leakage and thereby avoid collapse of the part during the hole forming operation.
In accordance with the present invention and for the piercing, for example, of a required round hole, the punch is provided with three distinctly different portions comprising (1) a piercing end portion at its piercing end having either a partial or completely circular cylindrical surface with a radius less than that of the hole required; (2) a hole-finishing portion having a completely circular cylindrical surface that is concentric with the piercing end portion and has a radius sized to produce the size of the hole required; and (3) a roll-forming portion joining the piercing end portion with the hole-finishing portion. The roll-forming portion has either (1) an acutely angled surface or (2) a rounded or convex surfaces and depending on the sealing that is required in a particular application of the punch according to the present invention, these surfaces extend either partially or completely about the periphery of the punch between the piercing end portion of the punch and the hole-finishing portion of the punch.
Immediately following piercing of the formed part by the piercing end portion and as the punch continues to be advanced into the part, the end portion then pilots the pierced region of the part onto the roll-forming surface which then forces the wall of the part about the perimeter of the punch to extrude and gradually roll-form a rolled edge portion in the part extending either partially or completely about the punch prior to the hole-finishing portion of the punch entering the part to complete the formation of the required hole.
The rolled edge portion in the part, whether extending partially or completely about the punch as described above, forces an extremely tight fit between the periphery of the punch and the formed part as the punch extends into the part. Resulting in a metal-to-metal seal between the formed part and punch of very high pressure sealing capacity that is immediately formed following the piercing operation and continues during completion of the hole forming process such that the high hydroforming pressure is maintained throughout the hole forming operation to prevent collapse of the part. This is quite advantageous in that numerous holes and of possibly various sizes and shapes can be pierced in a formed part with punches according to the present invention without causing any collapse or misshaping the part since the hydroforming pressure is maintained regardless of whether only one hole is pierced or many holes are pierced in the part.
Moreover, the amount of extrusion of metal into the interior of the part in the piercing of the hole can be made relatively small as compared with the extent of the seal effecting extrusions produced with prior known punches in an attempt to provide the required high pressure sealing. As a result, there is less likelihood for obstruction to the entry of a tool at an angle. With this advantage being most pronounced where the rolled edge portion needs to extend only partially about the punch in order to obtain the required high pressure sealing capacity. Thereby leaving an unrolled sidewall portion of the hole that presents no substantial obstruction.
Furthermore, the present invention can be quite useful in the ongoing development of certain metal working functions that can be employed in the hydroforming process. For example, in the hydrotrimming and hydroshearing of parts in the hydroforming process such as with the method and apparatus disclosed in U.S. Pat. No. 5,941,112 assigned to the assignee of this invention. Wherein the present invention in solving the leakage problem enables the maintenance of the very high hydroforming pressures required for the continuing development of such hydrotrimming and hydroshearing in the hydroforming process.
It will therefore be appreciated that the punch according to the present invention provides for piercing a hole in a hydroformed part during the hydroforming process while also performing a roll forming operation on the part that acts to prevent leakage past the punch that might otherwise occur and result in collapse of the part during the piercing operation. Wherein the roll forming action on the wall of the part may extend either partially or completely about the punch as the punch enters the part to thereby provide a very tight, high pressure seal as required between the part and the punch during the hole forming operation. And wherein the punch has an acutely angled surface or a rounded or convex surface for effecting this roll forming action on the wall of the part in producing the very tight, high pressure seal between the part and the punch during the hole forming operation.